This invention relates to a solid electrolytic capacitor capable of reducing leak current, a distributed constant type noise filter capable of reducing leak current, and a method of producing the same. More particularly, the invention relates to reduction of leak current in these components.
Referring to FIG. 1, a conventional solid electrolytic capacitor is configured to have an inner element 300, a resin package 8 formed by molding and covering the inner element 300, and anode and cathode terminals 9 and 10 electrically connected to the inner element 300 and partially exposed from the resin package 8.
The inner element 300 of the solid electrolytic capacitor is depicted in detail in FIGS. 2A, 2B, and 2C. The inner element 300 has an anode member 4 made of valve action metal and provided with an anode portion 4a and an anode lead portion 4b adjacent to each other. The inner element 300 further has a first dielectric layer 1 made of an oxide of the valve action metal and formed on lower and upper surfaces of the anode portion 4a, a second dielectric layer 3 made of an oxide of the valve action metal and formed on left side and right side surfaces of the anode portion 4a (in FIG. 2B) and an end side surface of the anode portion 4a (in FIG. 2C), and a cathode layer formed on the first and the second dielectric layers 1 and 3.
The cathode layer is composed of a conductive polymer layer 5 formed on the first dielectric layer 1 and the second dielectric layer 3, a graphite layer 6 formed on the conductive polymer layer 5, and a silver paste layer 7 formed on the graphite layer 6.
Again referring to FIG. 1, the anode terminal 9 is connected to a lower surface of the anode lead portion 4b of the anode member 4. The cathode terminal 10 is connected to a lower surface of the silver paste layer 7 of the cathode layer.
Referring to FIGS. 2a to 2C, a manufacturing process of the inner element 300 of the solid electrolytic capacitor is explained below.
First, a metal foil made of valve action metal of a relatively large size is prepared.
Lower and upper surfaces of the metal foil are enlarged in area by etching. Further, on an outer surface in a predetermined region of the metal foil, a dielectric layer made of oxide of the valve action metal is formed by an anodizing process with voltage applied. The metal foil with the dielectric layer is cut into many pieces each of which has a portion with the dielectric layer formed thereon and the other portion without the dielectric layer. In subsequent manufacturing, each of the pieces may be used as the anode member 4 with the first dielectric layer 1 partially formed thereon. The anode member 4 has a rectangular shape and is constituted of the anode portion 4a with the first dielectric layer 1 and the anode lead portion 4b without the first dielectric layer 1. However, the left side, the right side, and the end side surfaces of the anode portion 4a corresponding to cut surfaces of the anode member 4 (the piece) are uncovered by the first dielectric layer 1 and exposed.
Next, the second dielectric layer 3 is formed on the left side, the right side, and the end side surfaces of the anode portion 4a of the anode member 4 by an anodizing process with voltage applied.
Then, the conductive polymer layer 5, the graphite layer 6, and the silver paste layer 7 are formed in this order as the cathode layer on the first dielectric layer 1 and the second dielectric layer 3.
An applying voltage used in the anodizing process for the second dielectric layer 3 is set lower than that for the first dielectric layer 1 in order to prevent the first dielectric layer 1 from breaking-down in electrical isolation. Therefore, the second dielectric layer 3 is made thinner than the first dielectric layer 1. This means that the second dielectric layer 3 is inferior in electrical isolation to the first dielectric layer 1. When voltage is applied to the solid electrolytic capacitor (the element 300) in practical use, large leak current tends to pass through the second dielectric layer 3.
Referring to FIG. 3 together with FIGS. 4A, 4B, and 4C, a conventional distributed constant type noise filter is configured to have an inner element 500, a resin package 8, and first and second anode terminals 9 and 11 and a cathode terminal 10.
The inner element 500 of the distributed constant type noise filter has an anode member 4 made of valve action metal and provided with a first anode lead portion 4b, an anode portion 4a, and a second anode lead portion 4c in this order, a first dielectric layer 1 made of an oxide of the valve action metal and formed on lower and upper surfaces of the anode portion 4a, a second dielectric layer 3 made of an oxide of the valve action metal and formed on left side and right side surfaces of the anode portion 4a (in FIG. 4B), and a cathode layer formed on the first and the second dielectric layers 1 and 3.
The cathode layer is composed of a conductive polymer layer 5, a graphite layer 6, and a silver paste layer 7.
As shown in FIG. 3, the first and the second anode terminals 9 and 11 are connected to upper surfaces of the first and the second node lead portions 4b and 4c of the anode member 4, respectively. The cathode terminal 10 is connected to a lower surface of the silver paste layer 7 of the cathode layer.
Referring to FIGS. 4A to 4C, a manufacturing process of the inner element 500 of the distributed constant type noise filter is explained below.
First, a metal foil made of valve action metal of a relatively large size is prepared.
Lower and upper surfaces of the metal foil have been enlarged in area by etching. Further, on an outer surface in a predetermined region of the metal foil, a dielectric layer made of oxide of the valve action metal is formed by an anodizing process with voltage applied. The metal foil with the dielectric layer is cut into many pieces each of which has an intermediate portion with the dielectric layer formed thereon and two end portions without the dielectric layer. In subsequent manufacturing, each of the pieces may be used as the anode member 4 with the first dielectric layer 1 partially formed thereon. The anode member 4 has a rectangular shape and is constituted of the anode portion 4a with the first dielectric layer 1 and the first and the second anode lead portion 4b and 4c without the first dielectric layer 1. However, the left side, the right side, and the end side surfaces of the anode portion 4a corresponding to cut surfaces of the anode member 4 (the piece) are uncovered by the first dielectric layer 1 and exposed.
Next, the second dielectric layer 3 is formed on the left side and the right side surfaces of the anode portion 4a of the anode member 4 by an anodizing process with voltage applied.
Then, the conductive polymer layer 5, the graphite layer 6, and the silver paste layer 7 are formed in this order as the cathode layer on the first dielectric layer 1 and the second dielectric layer 3. Thus, the inner element 500 shown in FIGS. 4A to 4C has been manufactured.
An applying voltage used in the anodizing process for the second dielectric layer 3 is set lower than the anodizing process for the first dielectric layer 1 in order to prevent the first dielectric layer 1 from breaking-down in electrical isolation. Therefore, a similar problem arises with the electrical isolation property of the second dielectric layer mentioned above in connection with the conventional solid electrolytic capacitor.
Another example of a distributed constant type noise filter having the conductive polymer layer as the solid electrolytic layer is disclosed in Japanese Patent laid-open (JP-A) No. 2002-164760.
Further, a solid electrolytic capacitor having a second dielectric layer is disclosed in Japanese Patent laid-open (JP-A) Nos. Heisei 9-260215 and Heisei 10-74669. The second dielectric layer is formed by anodizing process with voltage applied on a cut surface of an anode member with a first dielectric layer formed thereon by anodizing process with voltage applied.
Further, a method of manufacturing solid electrolytic capacitor is disclosed in Japanese Patent laid-open (JP-A) No. Heisei 3-95910. In the method, a mask layer made of electrical insulating resin is formed on a unprocessed part of anodizing process of an anode member with a dielectric layer formed thereon.